This invention relates to communications systems and methods, in particular, to packet communication systems and methods.
Circuit emulation (CEM) systems, such as ATM CES, map native circuit frames received from a circuit into packets or cells. Sometimes this mapping is designed to minimize delay, as with ATM CES. Minimization of delay is accomplished by creating small packets, which minimizes the xe2x80x9ccapture delayxe2x80x9d. Capture delay is the time that it takes to acquire enough incoming circuit frames to create a packet. The drawback to minimizing capture delay is that the ratio of overhead (non-data control information) to data can be high, which leads to inefficient use of bandwidth. Other mappings are designed to increase efficiency and minimize overhead by increasing the number of transported frames, while holding constant the amount of overhead data. Reducing the percentage of overhead in this fashion has the disadvantage of increasing the capture delay.
Capture delay is one component in the round trip delay (RTD) for a packet to travel from one unit across the network to a second unit and then for a packet to return back from the second unit across the network to the first unit. The prior art has included means for measuring round trip delay, but these means have required the use of special test packets that were sent periodically. The use of periodic test packets adds to the overhead because these packets do not carry a CEM payload. The use of periodic test packets adds another tradeoff between having recent representative data on RTD and sending a large number of test packets without CEM payloads. The term payload is being use here and in the claims that follow to designate xe2x80x9creal dataxe2x80x9d in contrast with packet headers, various types of overhead for sending control data, and dummy data that is called xe2x80x9cfillerxe2x80x9d or xe2x80x9cstuffxe2x80x9d. Delivering real data (xe2x80x9cpayloadsxe2x80x9d) is the purpose for having a system, and everything else just facilitates that process.
Thus, prior art solutions have forced a fixed choice on the number of payload frames per CEM packet and thus a fixed choice between inefficient use of bandwidth or increasing the capture delay. A further shortcoming is that the prior art has not provided a method of collecting RTD while continuing to carry CEM payloads.
It is therefore an object of the invention to define a flexible mapping of circuits into packets. This method will allow flexibility in these dimensions:
The amount of data from a given circuit can be varied manually or automatically based on the measured end-to-end delay or round trip delay (RTD). The amount of data mapped to each packet is inversely proportional to the measured round trip delay.
If two or more circuits are destined for the same emulation endpoint, their data may be manually or automatically mapped into the same packet.
It is furthermore the object of this invention to provide a simple means of measuring RTD based on timestamps carried in a CEM packet that also conveys CEM payloads.
This disclosure provides a method for dynamically adjusting the number of data frames placed in a data unit or packet based on one or more recent measurements of round trip delay from the source device to a target device and back. Also disclosed is a method for measuring round trip delay by capturing certain relevant time values and transmitting these values within the packets carrying data frames so that new measurements of round trip delay can be made without the use of control packets that do not carry data frames.
Data structures for use with the disclosed methods are provided for a variety of protocols.